Can’t login or register or save or edit!

Please help! My site has worked fine for over a year. Then I accidentally installed what I think was a trojan on my computer from a link on my referal page of my stats. Suddenly I can’t save or edit any of my posts or pages. I can make other changes to my site such as changing the theme or turning on and off a plugin. I can even delete a post. I just can’t add one! Furthermore, if I log out I can’t log back in on that particular computer. Nor can anyone else even register to my site now. When trying to save or register, this message from Yahoo is what comes up everytime:

Sorry, Forbidden.
You don’t have permission to access this URL on this server. Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request. Please check the URL for proper spelling and capitalization.

Any ideas what is wrong? Do I need to completely reload my backup database?

Thanks so much!